No. Core features (payments, delivery, invoices, SSL, backups) are in every plan. ERP integration starts with STANDARD, B2B mode with PROFESSIONAL, and the REST API is only in ULTRA.
Yes. The platform has ready modules for MakeCommerce, Montonio, EveryPay, SEB, Paysera, Klix, Stripe, PayPal, Klarna, ESTO and others. You sign the contract directly with the provider you choose.
Yes. In the design builder you change layout, colours, fonts and blocks. If you need more, you can add your own CSS and JS, and updates won't overwrite it.